1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image based on image data, and a control method thereof.
2. Description of the Related Art
Ordinarily, in a printer employing an electrophotographic method used in a laser beam printer, a digital copy machine, or the like, the surface of a photosensitive drum is raster-scanned with a laser beam that has been modulated based on an image signal to form an electrostatic latent image, and that latent image is developed and transferred to a recording paper (sheet).
In a printer capable of printing on both faces of a sheet, when an image is formed on the front face of a sheet and fixed by applying heat, due to a change in the amount of moisture included in the sheet, the size of the sheet contracts or expands. When, in this state, an image is formed and fixed on the back face of the sheet. In such way, the sheet on which the double-sided printing is performed is discharged from the printer and the amount of moisture of the sheet has returned to its original state, the size of the image formed on the back face differs from the size of the image formed on the front face, so there is the problem that scaling is not the same for the image on the front face and the image on the back face.
To address this problem, a method has been proposed for appropriately correcting the scaling in a main scanning direction, which is the direction in which the laser beam scans, and the lengthwise direction of the photosensitive drum. In this method, an image signal is modulated to pixel data in which the image signal is divided in pixel units into a plurality of bits arranged in the main scanning direction of the laser beam, and one or more correction points (pixels) on one line are determined. The last bit of segmentation-modulated pixel data of a pixel positioned before the correction point is added as the first bit of segmentation-modulated pixel data of a pixel positioned at the correction point. Then, segmentation-modulated pixel data of successive pixels relative to each pixel positioned at and after the correction point is moved to the next pixel in bit units (see Japanese Patent Application Laid-open No. 2004-35190).
However, the method in the above conventional example is appropriate for correcting the scaling in the main scanning direction but it is not suitable for correcting the scaling in a sub-scanning direction. That is, in order to perform segmentation modulation, it is necessary to have the ability to form a high resolution image relative to the resolution before segmentation modulation. For example, where segmentation modulation is 1/16 (one pixel is modulated to 16 bits), a resolution of 16 times the resolution prior to segmentation modulation is necessary. In order to realize this, it is necessary to greatly increase the number of beams of a semiconductor laser, the number of revolutions of a drive motor of a polygonal mirror, the number of faces of the polygonal mirror, or the like. This results in increased costs for the image forming apparatus.